From GNUstepWiki

Pbxbuild (not to be confused with the OS X tool of the same name) is used to take Xcode project files and build the code. It does this by creating GNUmakefiles, and using Make to build those.

It reads .xcodeproj or .xcode files and parses them to determine the structure. Then it re-creates that structure with a makefile. It does not inspect the source code, so it cannot guarantee that an OS X project will compile on GNUstep.

Hans Baier created pbxbuild in 2006, and it's distributed with the GNUstep tools.

Pbxbuild is now deprecated ([1])